Are you sure c:\ is actually indexed? It's not by default.

Number of files will make a large difference but a 5:1 ratio not so much. Windows has a lot of files. My C:\ (only drive) is 335k files.

Windows 7 introduced more rigorous searching I think and actually searches in the files - not sure if by default but I suspect not.

Check
http://www.sevenforums.com/tutorials/36098-search-options-change-restore.html
http://www.dummies.com/how-to/content/how-to-customize-windows-7s-search-settings.html

If properly indexed though, it should be fast. Even then an SSD is still going to be faster in like for like searches